Calcium sulfate exists in the earth's sediment in three mineral forms. The most common is gypsum which is the fully hydrated form: CaSO.sub.4.2H.sub.2 O. The partially hydrated form of the mineral is known as bassanite and has the following formula (CaSO.sub.4).sub.2.H.sub.2 O.
Calcium sulfate crystals with no water hydration are known as "anhydrite". When found in sedimentary rocks, it is a compact, granular mass which resembles marble in appearance. It differs from gypsum and bassanite in hardness and lack of hydration.
For many years the origin of anhydrite in ancient sedimentary rocks has been controversial. Probably the majority of sedimentologists today have concluded that most ancient anhydrites results from dehydration of primary gypsum. This conclusion is based on the apparent impossibility of producing synthetic primary anhydrite under laboratory conditions similar to those prevailing within evaporite environments, and on the rarity of anhydrite in modern evaporite sediments. It has been known that anhydrite can be made from gypsum by heating natural gypsum to elevated temperatures as high as 800.degree. C. to 1000.degree. C. to drive off the water, leaving a secondary formation of anhydrite. However, such processes are time consuming, energy consuming and therefore not economical.
There is presently no economically practical source of synthetic precipitated anhydrite. The phrase "synthetic precipitated anhydrite" is used to designate anhydrite which is available from a controlled chemical manufacturing process. This is to be distinguished from anhydrite available from natural sedimentation processes, or that formed by either natural or commercial dehydration of gypsum.
It is a primary object of this invention to provide a commercially feasible process for preparation of precipitated synthetic anhydrite, on a bulk basis.
It is a further object of this invention to provide a process for the commercially feasible preparation of anhydrite which utilizes moderate conditions of temperature and pressure resembling those found in hot, arid regions.
It is a further object of this invention to provide a process for the preparation of anhydrite so that large quantities of anhydrite might be available for a multitude of probable uses such as fillers, pigments, whitening agents, and use as extenders, and the like.
In nature, as in laboratory experiments, anhydrite can potentially form by a variety of mechanisms. Undoubtedly, much anhydrite in ancient rocks resulted from dehydration of gypsum or bassanite. Although these minerals are easily dehydrated at high temperatures under laboratory conditions, there is some uncertainty about simple dehydration in aqueous solutions. For example, some workers have observed that gypsum dehydrates to bassanite via a solution-precipitation mechanism. The major geochemical unknown, however, is whether anhydrite can actually precipitate from solutions in a sedimentary environment. This precipitation could involve two different types of crystal nucleation; primary or secondary. In primary nucleation, spontaneous crystallization occurs in the absence of crystalline seed material. In secondary nucleation, crystallization is induced by crystalline seeds that may be, but are not necessarily, the same as the nucleating substance. An example of secondary nucleation is the crystallization of anhydrite in the presence of anhydrite seeds, or in the presence of gypsum seeds that slowly dissolve as anhydrite simultaneously grows.
Despite much experimentation over many years, researchers have never succeeded in producing primary nucleation of anhydrite under comparatively low temperature conditions, that is, conditions even remotely similar to those occuring in natural sedimentary environments. In the experiments, gypsum or bassanite invariably precipitate metastably at temperatures and water salinites in which anhydrite is the expected stable phase. Previous experiments have induced the secondary nucleation and growth of anhydrite in the presence of gypsum or bassanite precursors only under conditions of extremely high temperatures, salinites, and/or reactant ion concentrations. These conditions rarely, if ever, are duplicated within natural sedimentary environments, and for the most part are not commercially practicable.
The primary objective of the present invention, as heretofore stated, is to form anhydrite, at temperatures and inorganic ion concentrations, common in waters located in hot, arid, regions of the earth, and at conditions which will readily allow commercial manufacture.